The present invention relates to a process for producing cane sugar from an unpurified or partly purified aqueous cane sugar composition, such as cane juice or dissolved raw sugar from a mill.
The production of cane sugar for human consumption generally comprises two distinct operations, namely the production of raw sugar and the production of refined sugar, which are often carried out in separate locations. Production of raw sugar typically takes place at a sugar mill, which is usually located in or near sugar cane fields. In the mill sugar cane stalks are chopped into pieces and the pieces are crushed in a series of mills in order to remove the juice. The juice from the first set of roller mills is referred to as "first juice," while the total juice from all the roller mills in the process is referred to as "mixed juice." The juice is normally limed, deaerated and clarified (i.e., removal of suspended solids, usually by sedimentation). The clarified stream is referred to as "clarified juice." The juice is then evaporated to a thick syrup (known as "evaporated juice"), and crystallized in a vacuum pan. The "massecuite" (i.e., mixture of sugar syrup and crystals) produced in the vacuum pan is stirred in a crystallizer, and the mother syrup is spun off from the raw sugar crystals in a centrifugal separator. The solid sugar in the centrifugal basket is washed with water to remove remaining syrup. The solid crystalline product is termed "raw sugar."
Depending on the exact nature of the process steps and conditions used in the sugar mill, the raw sugar product can be made more or less pure. A more highly purified mill product is sometimes referred to as "Mill White" or "Plantation White" sugar. The production of these sugars requires sulphitation before or after clarification using SO.sub.2 gas. It usually requires a second clarification step, usually at the syrup stage and sometimes a second sulphitation step. In nearly all cases the ash content of this sugar is much higher, perhaps by more than four times, that of refined white sugar. Although these particular mill products can be sold for human consumption without further processing in some instances, generally raw sugar must be further refined before it reaches a commercially acceptable level of purity, particularly for subsequent use by food and drink manufacturers.
Therefore, the raw sugar from a mill is usually transported to a sugar refinery for further processing. In a conventional cane sugar refining process, the raw sugar is first washed and centrifuged to remove adherent syrup, and the "affined sugar" thus produced is dissolved in water as "melter liquor." The syrup removed from the surface of the raw sugar is known as "affination syrup" and is broadly similar in composition to the mother syrup from the raw sugar crystallisation. The affination syrup is processed through vacuum pans, crystallizers and centrifugal separators similar to those used for the production of raw sugar, to recover an impure crystalline sugar product which has approximately the same composition as raw sugar. This recovered sugar product is dissolved in water, along with the affined raw sugar, to make melter liquor. Thus, the treatment of affination syrup in the recovery house of the refinery is somewhat similar to the production of raw sugar from evaporated juice.
The melter liquor is then purified, generally by the successive steps of clarification (also referred to as "defecation") and decolorization, and the resulting "fine liquor" is crystallized to give refined sugar. The clarification step usually involves forming an inorganic precipitate in the liquor, and removing the precipitate and along with it insoluble and colloidal impurities which were present in the melter liquor. In one of the clarification processes commonly used for melter liquor, termed "phosphatation," the inorganic precipitate is calcium phosphate, normally formed by the addition of lime and phosphoric acid to the liquor. The calcium phosphate precipitate is usually removed from the liquor by flotation, in association with air bubbles. Other clarification processes, termed carbonation (or carbonatation) processes, involve adding lime and carbon dioxide to the liquor, and produce calcium carbonate precipitate.
The geographical separation of cane sugar milling and refining operations results in increased overall manufacturing cost. It is not practical to build a refinery at the site of every cane sugar mill, due to the relatively large capital cost of conventional refining process equipment.
The juice produced in a cane sugar mill typically has a color of about 14,000 icu, and conventional mill technology can process this to raw sugar with a whole color of about 3,000 icu, and a well affined color of 400-800 icu. It is very difficult to produce white sugar of less than 80 icu in one crystallization in a mill because of the extremely high colors of the starting material, and because it is difficult to filter cane juice or syrup. After a crystallization at the mill, a significant portion of colored materials are concentrated in the raw sugar crystals, and therefore a high degree of decolorization is required at the refinery in order to produce white sugar.
One process that has been used in an attempt to overcome this problem is referred to as the Java process. A juice stream in a cane sugar mill is treated with an excess of lime, usually at least equal to about 10% by weight of the sugar in the juice. Excess lime is removed with carbon dioxide. This process evolved into the deHaan process, which used milk of lime and carbonation, at 55.degree. C. The deHaan process used multiple incremental additions of milk of lime followed by carbonation. These processes did improve the color of the crystallized sugar product from the mill, but the very large amount of lime required in order to achieve good filtration made the processes economically undesirable, as well as needing a large amount of filtration equipment, and producing a large amount of material that would need to be disposed of, giving environmental problems.
There is a need for improved cane sugar processes that would allow production of is a highly purified product using a single crystallization, and preferably in a single plant, rather than in separate sugar mills and refineries, in order to reduce the cost and simplify the processing of cane sugar for human consumption.